The present invention relates to a hydraulic driving arrangement for a machine element intended for processing a workpiece and performing to this end an operating cycle composed of a rapid feed motion directed towards the workpiece, followed by a working stroke effected in the same direction and serving to process the workpiece, and finally an oppositely directed rapid return motion, with a hydraulic cylinder serving as driving element and comprising at least three working surface A1, A2 A3 defining each one delimiting face of a first, a second and a third pressure chamber, the rapid feed motion and the rapid return motion of the piston of the hydraulic cylinder for the machine element being controllable by alternate admission and release of pressure to and from the first and the second pressure chambers of the hydraulic cylinder, while the feeding power can be increased, if this should become necessary for performing the working stroke, by admitting pressure to the third pressure chamber of the hydraulic cylinder delimited by the said third working surface A3.
Hydraulic driving arrangements of this type have been generally known, for instance in connection with punching machines, in which the punching tool mounted on the piston of the hydraulic drive cylinder approaches the workpiece as closely as possible at rapid speed and relatively low feeding force, then penetrates and cuts the workpiece at increased feeding force, ejects the piece of material cut out in the further course of its working stroke through the punched opening, and finally returns to its initial position at rapid speed.
The following points are, however, problematic in these arrangements:
1. The exact change-over from the rapid feed to working stroke, i.e. the change-over from low to high feeding power, which is achieved by alternate or additional admission of pressure to the third pressure chamber of the hydraulic cylinder delimited by the large piston surface A3; and PA0 2. the reversal of the direction of movement of the tool which is achieved by alternating the admission of pressure between the first and the second pressure chambers of the hydraulic cylinder.
If the change-over from rapid feed to working stroke is effected as a function of the length of travel, for instance by means of suitable approximation switches responding to specific instantaneous positions of the piston of the hydraulic cylinder, whose output signals actuate control valves taking the form of solenoid valves, a change-over from rapid feed to working stroke will be effected in each operating cycle, regardless of whether or not it is actually required, so that relatively long cycle times must be accepted which imposes considerable and often unnecessary limitations upon the working speed of a punching machine which is expected to perform as many working cycles per minute as possible.
This disadvantage is aggravated by the fact that solenoid valves that can be electrically actuated require switching times in the range of approx. 20 to 25 ms, which again contributes to extending the cycle times. The same disadvantage must be accepted in cases where the change-over from rapid feed to working stroke is effected in response to the pressure and, thus, in a manner more likely to meet the practical requirements, if the means used for effecting the change-over consist in electromagnetic push-button switches which respond to the pressure supplied to the pressure chamber of the hydraulic driving cylinder in the rapid motion and whose electric output signals actuate solenoid valves to supply the appropriate pressures to the pressure chambers of the hydraulic drive cylinders.
As regards the reversal of motion of the piston of the drive cylinder controlled in this manner it is further a considerable disadvantage that the reversal of the direction of action of the forces acting upon the piston occurs necessarily in a sudden manner which results in vibrations which cause not only increased wear, but also heavy operating noises.
Although it is certainly possible to give the hydraulic drive cylinder and the machine equipped therewith a stability sufficient to achieve the minimum service life deemed necessary, and to equip the hydraulic drive cylinder with additional hydromechanical and/or resilient damping elements in order to reduce the vibrations which would otherwise be encountered at the dead centers of its advance and return movements, so as to avoid excessive noise, the technical input required for this purpose is quite considerable and leads finally again to increased cycle times and also to the necessity to increase the installed drive power.
Now, it is the object of the invention to provide a hydraulic drive system of the type described above, which makes it possible, on the one hand, to obtain clearly reduced cycle times for a machine equipped with this device and ensures, on the other hand, that the driven machine element enters and/or passes through, its different dead center positions or end positions during any working cycle smoothly and without causing excessive vibrations.
According to advantageous features of the present invention, a hydraulic driving arrangement for a machine element intended for processing a workpiece and performing an operating cycle composed of a rapid feed motion directed towards the workpiece, followed by a working stroke effected in the same direction and serving to process the workpiece, and finally oppositely directed rapid return motion is characterized in that, for motion control of the machine element with respect to direction and lift, a hydraulic control circuit is provided, with the hydraulic control circuit comprising a hydro-mechanical actual-value, fee-back device adapted to be supplied with presetting signals characteristic of at least end positions of the machine element for presetting the desired values. The hydraulic control circuit effects an alternative supply of pressure to first and second pressure chambers and, if necessary, the supply of pressure to a third pressure chamber. For changing over the hydraulic cylinder from rapid feed motion to the working stroke, a reversing valve with a hydraulic pilot valve is provided which, in a first flow position corresponding to the rapid-feed motions, connects the third pressure chamber of the hydraulic cylinder with a tank of a pressure supply source. In a second flow position, connects the same pressure chamber with an A pressure outlet of a final control element of the control circuit. For controlling the reversing valve, a pilot control valve arrangement is provided which responds to output pressure P.sub.A of the hydraulic control circuit and which moves a reversing valve into a second flow position when the output pressure P.sub.A exceeds a pre-determined threshold value P.sub.s1, and returns the reversing valve into the first flow position when the output pressure P.sub.A of the control circuit has dropped to a value P.sub.s2 corresponding maximally to a value P.sub.s1 .multidot.Al/A.sub.L, for in: A.sub.1 is a size of a surface of a piston upon which the output pressure P.sub.A of the hydraulic control circuit acts during rapid feed motion of the hydraulic cylinder, and A.sub.L is a size of a overall surface of a piston upon which the controller output pressure P.sub.A acts during the working stroke of the hydraulic cylinder.
In this arrangement, motion control is effected by a control circuit which ensures that the piston surface and/or the pressure chambers utilized for developing the needed power are at any time, i.e. during both the advance and the return movements of the hydraulic cylinder, supplied with the pressure required and that, when the piston approaches its end position determined by the given set value, the pressure in the said pressure chambers is reduced so that the piston enters its end positions smoothly and without causing undue vibrations. As a result thereof, the control circuit which uses a mechanical nominal-value feedback device, has a favorable high control frequency so that an effective control of the pressures, in the meaning of a steady decrease of the moving speed of the piston of the hydraulic cylinder towards its end positions, is ensured even in the case of relatively short cycle times. In order to make the best possible use of the favourably high control frequency of this mechano-hydraulic control circuit, the supply of pressure to, and release of pressure from the second pressure arm, through which the feeding power can be increased of reduced as needed, is controlled by means of a reversing valve with hydraulic pilot valve and short switching times in the first flow position of which i.e. in the position associated with the rapid feed motion, the further pressure chamber communicates with the tank, while in its second flow position, i.e., in the position associated with the working stroke, the further pressure chamber of the hydraulic drive cylinder communicates with the pressure outlet of the control circuit so that the pressures of all pressure chambers to which pressures are supplied with a view to developing the feeding power are subjected to the pressure control also during the working stroke which means that the motion remains as vibration-free as possible even when the piston of the hydraulic drive cylinder enters any of its end position under load. In order to enable the reversing valve itself to be moved with sufficient rapidity from its first to its second flow position, and vice versa, a pilot valve arrangement is provided which is hydraulically operated and which thus also offers favourably short switching times. The said pilot valve causes the reversing valve to return automatically to its second flow position when the output pressure of the control circuit exceeds a pre-determined threshold value p.sub.s1, and to re-assume its first flow position as soon as the output pressure of the control value has dropped to a value p.sub.s2 not greater than the value p.sub.s1 .multidot.A.sub.1 /A.sub.L, wherein A.sub.1 is the size of the surface upon which the output pressure P.sub.A of the control circuit acts in the rapid feed motion of the hydraulic cylinder, and A.sub.L is the size of the full surface of the piston upon which the output pressure of the control circuit acts in load operation of the hydraulic cylinder. This ensures that pressure is supplied to the further pressure chamber only as and when an increased feeding power is required, whereas otherwise the rapid-motion conditions of the piston are utilized as far as possible to achieve optimally short cycle times. This also minimizes the total energy requirements of the drive so that the device of the invention requires only a relatively low installed drive power, at least on the condition that an accumulator pumping system is used for supplying the required pressure.
Advantageously, the lower pressure threshold value p.sub.s2 at which the drive system of the invention is switched over from working stroke to rapid motion, may be lower by a defined amount than the value p.sub.s1 A.sub.1 /A.sub.L, so that when switching over from the rapid motion to working stroke the initial reduction of the output pressure P.sub.A of the control circuit does not immediately switch the system back to rapid motion when for instance the feeding power required in load operation is only little greater than the feeding power developed during the rapid motion when the pressure acts only on the working surface A.sub.1. Such a design ensures a largely uniform motion sequence also in cases where a relatively small increase of the feeding power is required only for the working stroke.
To achieve an advantageously simple construction of a pilot valve arrangement suited for achieving a desirably quick reversal of the reversing valve, in accordance with the functional requirements, according to the invention, an output stage of a pilot control valve arrangement is formed by a 3/2 directional valve constructed as a pressure-control sliding valve with a first and second control pressure chamber. When the pressure level in the two control pressure chambers is substantially equal, a piston of the 3/2 directional valve is held by a bias of a restoring spring in a first upwarding position corresponding to a neutral position in which a high level pressure signal is applied to its output which retains a reversing valve in a first flow position corresponding to the rapid feed motions of the hydraulic cylinder, while when the pressure prevailing in the first control pressure chamber is higher than that prevailing in the second pressure chamber, it is moved into a second upwarding position in which the control pressure chamber of the reversing valve is pressure-relieved or connected with the tank of the pressure source so that the reversing valve is moved into a flow position associated with the working stroke. The first control pressure chamber of the 3/2 directional valve is directly connected with A pressure outlet of the control circuit and the second control pressure chamber of the 3/2 directional valve is connected with the A pressure outlet of the control circuit through a flow resistance. An over-center device responds to the output pressure P.sub.A of the control circuit and is constructed as a proportioning pressure regulator provided which, when the output pressure P.sub.A exceeds the first threshold value P.sub.s1 connects the second control pressure chamber of the valve with the tank and, when the output pressure P.sub.A of the control circuit drops below a lower pressure value P.sub.s2, cuts off this connection between the control pressure chamber and the tank of the supply pressure source.
Arrangements comprising a 3/3 directional valve as an output stage, and an over-center device taking the form of a proportioning pressure regulator and employed as a pilot valve for the 3/2 directional valve is extremely advantageous. The features of this valve, which preferably is constructed as a seat valve, and its functional connection to a pilot valve such as contemplated by the present invention is extremely advantageous. More particularly, in a hydraulic drive system of the present invention, the lower threshold value P.sub.s1 at which the pilot control valve arrangement moves into the first upwarding position corresponding to the first flow position of a reversing valve may be defined by the formula: EQU P.sub.s2 =P.sub.s1 (A.sub.1 /A.sub.L).multidot.q),
wherein:
0.95&gt;q&gt;0.8.
In accordance with further features of the present invention, a bias of a pressure spring provided for urging the loaded valve body of the valve seat into a closed position may be adjustable. The bias of the pressure spring urging the valve body of the seat valve of the over-center device into its closed position can be adjusted which makes it easy to pre-determine the threshold value P.sub.s1 at which the drive system is to switch over automatically from rapid motion to load operation.
According to the present invention, a seat valve may be provided which includes a ball valve having a ball of a diameter smaller than a diameter of a housing bore in which the ball is arranged to move in an axial direction. A pressure piston is slidable guided in the housing and urged against the ball by a pressure spring bearing against the ball by a conical centering face. This arrangement of an over-center device favorably guarantees favorable short response times of the over-center device and/or the pilot valve arrangement.
In accordance with the further features of the present invention, the reversing valve is constructed as a 3/2 directional sliding valve which is shifted into a first flow position by a high-level output pressure of the pilot control valve against a restoring force of a pressure spring and moved into a second flow position by a restoring force of a pressure spring at a low output pressure level. An output pressure chamber of the reversing valve, which remains the same in all operating positions of the latter in which communicates through a flow path of low flow resistance with the third pressure chamber of the hydraulic cylinder is, in the first flow position of the reversing valve connected with the tank of the tank of the pressure source through a flow path of likewise low flow resistance. In the second flow position of the reversing valve in which the first mentioned flow path is blocked by the fact that a sealing edge of the valve body bears against a conical valve seat of a valve housing, is connected with the A pressure outlet of the hydraulic control circuit by a control channel in a valve housing which is opened in this position of the valve body. This general structure of a reversing valve is suited for the purposes of the drive system of the invention which in certain preferred embodiments of its different flow positions.
According to the present invention, the valve housing of the reversing valve includes a valve bore constructed as a step bore having a narrower step communicating within the conical valve seat with the third pressure chamber of the hydraulic cylinder and a larger bore step comprising a control channel communicating with the pressure outlet of the hydraulic control circuit. The valve body of the reversing valve is constructed as a substantially tubular body slidable guided, in a pressure-type relationship, in the valve bore by contact of an outer surface thereof with a wall of the narrower step of the bore and contact of an outwardly projecting flange with the wall of the larger step of the valve bore. The flange in annular face between the two bore steps delimit the control pressure chamber the reversing valve in an axial direction, and the flange acts to shut off the control channel against the output pressure chamber in the first flow position of the reversing and to open the connection between the control channel and the output pressure chamber of the reversing valve in the second flow position.
A large-volume annular space forming a part of the tank of the pressure supply source, according to the present invention, is arranged in the housing of the reversing valve in a coaxial arrangement with the valve body, with the annular space being adapted to be connected with the output pressure chamber of the reversing valve through large radially extending overflow channels which open into the narrower bore step and which are open in the first flow position of the reversing valve and closed in the second flow position by the valve body.
To provide for favorably low resistance and simply construction as well as to realize a compact unit incorporating the hydraulic drive cylinder, according to the present invention, the reversing valve forms an axial extension of the hydraulic cylinder, a housing of the hydraulic cylinder and valve housing of the reversing valve formed one single constructional unit.
Advantageously, according to the present invention, an output stage of the hydraulic control circuit includes a conventional mechano-hydraulic follow-up control valve including a 4/3 directional valve with a pre-setting arrangement including a spindle drive and an actual-value back-fitting device, with presetting being effected by rotary movements of a spindle nut by rotary angles .zeta..sub.V and .zeta..sub.R correlated, with respect to amount and direction, and with feed and return travel of a piston of the hydraulic cylinder and back-feed of different actual piston positions be effected by a mechanical back-feeding device which causes a spindle of the spindle drive to perform rotary movements correlated, with respect to mount and direction, with the feed and return movements of the position of the hydraulic cylinder.
By virtue of the last noted features, a follow-up control valve of generally conventional construction is particularly suited for use as an output stage of the control circuit controlling the motions of the hydraulic cylinder. The valve is suited for both digital and analog pre-setting of the desired motion sequences and strokes.
For presetting the feed and return motions of the piston of the hydraulic cylinder, according to the present invention, a stepping motor capable of being controlled in a start-stop operation is provided which can operate at a control pulse frequency of being twenty to one-hundred times greater than a number of stepping control pulses required within a period of time of a work cycle for achieving a sufficiently exact motion control.
These last noted features define a construction of the pre-setting device of the control circuit suited for providing a stored or programmable control for the operating cycles, which opens up the most diverse applications for the drive system of the invention. In this arrangement, motion can be controlled either by the preset value leading, by one or just a few setting steps, the actual value of the instantaneous position of the piston as registered by the feedback device, or simply by a preset value corresponding to the overall stroke in the feed or return direction being given at the beginning of the feed movement or the return movement phase of the piston within a period of time that is small compared with the duration of these movement phases; the latter of these two types of motion control allows particularly short work cycle times to be achieved.
The general lay-out of a pre-setting device especially designed for this manner of pre-setting the end position, which insofar can be regarded a viable alternative. A considerably simpler design construction is realized with the provision of an electro-hydraulic presetting mechanism within the hydraulic control circuit for a motion control of the piston of the hydraulic cylinder, with the electro-hydraulic presetting effecting presetting of the desired end position values determining the feed and return motions of the piston of the hydraulic cylinder by causing a double-acting control cylinder in a cycle-relating manner to move its piston into alternative end positions associated with defined rotary positions of the spindle nut of the follow-up control valve.
Advantageously, according to the present invention, for controlling the control cylinder of the pre-setting mechanism, a 4/3 directional solenoid valve with two conical windings is provided. In a non-excited condition of the windings, the 4/3 directional solenoid valve assumes a neutral blocked or zero position associated with a neutral position of the piston of the control cylinder, while by alternatively exciting the windings by a control current the valve can be moved against a restoring force of pressure springs to its alternative flow positions, in which the piston of the control cylinder moves into its alternative end positions. A control stage is provided which responds to the output signals of end position pickups generating output signals characteristic of one or the other end positions of the control piston and to output pulses of a pulse generator provided for controlling the cycle and which generates necessary control current pulses for controlling the 4/3 directional solenoid valve in an appropriate manner by logic combination of the input signals.
Advantageously, the end position pickups include at least two approximation switches which, when occupying an position opposite a triggering finger which follows the movements of the piston of the control cylinder generates output signals characteristic of a given position, for instance, a high-level voltage signal, the end position pickups being slidable mounted on a guide element extended in parallel to a direction of movement of the triggering finger and arranged to be fixed at a selective distance from each other corresponding to the end position of the piston. The triggering finger is mounted, if necessary, for being displaceable on a piston rod projecting from a housing of the control cylinder and for being fixed thereon.
Advantageously, output signals of the actual positions emitted by the end position pickups are high-level voltage signals in the end positions of the control piston and output pulses of the pulse generator are also high-level voltage signals for a duration of successive feed and return motions of the cylinder, while for the rest of the time the same signals are low-level voltage signals. The control stage advantageously comprises a first storage circuit that can be set to high output signal levels by rising flanks of the output signals of the first end position pickup and reset by the rising flanks of output pulses of the second end position pickup. A second storage circuit can be set to high output signal levels by the rising flanks of the output pulses of the second end position pickup and reset by the rising flanks of the output pulses of the first end position pickup. A third storage circuit can be set to high output signal levels by dropping flanks of output pulses emitted by a pulse generated and reset by rising flanks of output pulses of the first end position pickup. A first AND gate with two inputs is provided to which output signals of the first storage circuit and output pulses of the pulse generator are applied as input pulses. A second AND gate with two inputs is provided to which the output pulses of the second and third storage circuits are applied as input signals. The output pulses of the two AND gates with two inputs can release the current control signal for controlling the 4/3 directional solenoid valve.
The presetting device of the present invention is particularly suited for punching machines for a plurality of operating cycles follow each other in rapid succession.
Considering that in the drive system of the invention the forces effective during the feed and/or return motions of the piston of the hydraulic cylinder are obtained by a controlled supply of pressure to the active working surfaces A.sub.1 and/or A.sub.3, or A.sub.2, respectively, a pressure drop will be encountered in the pressure chambers of the hydraulic cylinder to which pressure is supplied via the pressure output of the control circuit, or balancing of the pressures active in the different pressure chambers, will be encountered each time the piston reaches one of its end positions. The invention therefore provides a monitoring system which responds in a characteristic manner to the pressures prevailing in the individual pressure chambers of the hydraulic cylinder, and by means of which it is easy to ascertain whether or not the piston of the hydraulic cylinder reaches the end positions corresponding to the pre-set values in the course of an operating cycle, and to derive therefrom information on the proper or incorrect, or insufficient operation of the drive system. If, for instance, the output pressure of the control system remains at high level after the drive system has changed over from rapid motion to working stroke, this is a safe indication that the piston of the hydraulic drive cylinder cannot complete its working stroke, either because, in the case of a punching die, the die may have become blunt, or because, in the case of a pressing or stamping die, the workpiece cannot be shaped as required, for instance because it is not properly supported. Insofar, the monitoring device may be used to indicate malfunctions of the machine. On the other hand, it is also possible to conclude from the pressure drop and/or equalization of pressure caused by the pressure control that the hydraulic cylinder has completed its working stroke and is approaching its end position, so that a corresponding output signal of the monitoring system can be used to trigger the pre-setting process for the return stroke already before the piston has actually completed the working stroke. In this manner, the working cycle times, for instance for repeated punching operations, can be still further reduced.
More particularly, according to yet further features of the present invention, a monitoring device is provided which responds to pressure in the first or the third pressure chambers and to pressure in the second pressure chamber of the hydraulic cylinder, and which generates a characteristic output signal as long as force is acting in the feed or return directions of a piston of the hydraulic cylinder are greater than certain predetermined threshold values.
The present invention also provides for a monitoring system using a double-acting differential piston wherein the ratio between the active working surfaces is equal to the ratio between the working surfaces of the piston of the hydraulic drive cylinder used for generating the forces acting in the opposite direction.
According to the present invention, the monitoring device includes at least one double-acting hydraulic cylinder having a piston defining a secondary pressure chamber which communicates with the first or second pressure chambers of the hydraulic drive cylinder against a second secondary pressure chamber which communicates with the second pressure chamber of the hydraulic drive cylinder, with the piston including a step piston having piston surfaces of larger and smaller piston steps which corresponds to cross-sectional surfaces of the secondary pressure chambers and exhibit the same ratio as effective cross-sectional surfaces of the connected pressure chambers of the hydraulic cylinder. The piston can be displaced against an increasing restoring force of an equilibrium position defined by a position between possible end positions.
In order to provide for a differential piston having a preferably miniaturized construction, according to the present invention, the piston surfaces of the stepped piston defining the secondary pressure chambers of the hydraulic cylinder of the monitoring device are much smaller than the effective piston surfaces of the piston of the hydraulic drive cylinder which define one side of the pressure chambers communicating with the secondary pressure chambers of the hydraulic cylinder of the monitoring device.
Advantageously, according to the present invention, a surface ratio of the surfaces of the stepped position defining the secondary pressure chambers of the hydraulic cylinder of the monitoring device, to surfaces of the piston of the hydraulic drive cylinder delimitating the pressure chambers of the hydraulic drive cylinder communicate with the secondary pressure chambers in a range between one/one hundred and one/two thousand and, preferably, one/one thousand.
This arrangement offers the advantage that the displacements of the differential piston of the monitoring system are always proportionate to the forces acting upon the piston of the hydraulic drive cylinder in the direction of the feed or return motion, so that when the differential piston is coupled with an analog displacement pickup, continuous registering of the forces generated within the drive system becomes possible.
For a great number of applications it will, however, suffice to use a monitoring system wherein at least two end position pickups connects with the double-acting hydraulic cylinder, the first of the end position pickups generating a characteristic output signal when the stepped piston is in one end position which is associated with an excessive pressure P&gt;P.sub.s1 in the first secondary pressure chamber of the hydraulic cylinder of the monitoring device. The second end position pickup generates a characteristic output signal when the step piston is in its outer end position associated with excessive pressure in the second secondary control chamber of the hydraulic cylinder of the monitoring device.
A first monitoring device is provided whose first secondary pressure chamber communicates with the first pressure chamber of the hydraulic drive cylinder and a second monitoring system whose first secondary pressure chamber communicates with the third pressure chamber of the hydraulic drive cylinder while the second secondary pressure chamber of the monitoring devices communicate with the second pressure chamber of the hydraulic drive cylinder.
With such an arrangement, it is possible to register only the end positions of the differential piston and/or to monitor the maximum forces acting in the feed and return directions of the hydraulic cylinder.
Features described by claims 21 and/or 22, which is designed to register only the end positions of the differential piston and/or to monitor the maximum forces acting in the feed and return directions of the hydraulic cylinder.
To define particularly advantageous applications for a drive system in accordance with the present invention, a hydraulic drive system may be utilized in punching or nipple machines for raid succession of work cycles and performance of three hundred to six hundred work cycles per minute. It is also possible for the hydraulic drive system of the present invention to be utilized in presses or stamping machines.